This invention relates generally to the rendering of latent electrostatic images visible and, more particularly, to a single component developer apparatus including a donor roll and an improved device for reloading the donor roll with toner.
The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoconductive insulating surface or photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not struck by radiation.
This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
The development of images by various methods, including electrostatographic means is well known. In some of these systems, toner particles are deposited on an electrostatic latent image contained on an insulating surface, such as selenium, utilizing, for example, cascade development, magnetic brush development, powder cloud development, touchdown development, and the like.
In view of some of the disadvantages of two component development systems, there has been considerable effort directed to designing single component development systems which utilize only toner particles, for example, U.S. Pat. No. 2,846,333, which discloses a single component developer composition that is comprised of toner resins, colorants and magnetic materials. Many of the single component development systems contain conductive toner particles, whereby imagewise toner deposition onto the imaging member is obtained by induction charging of the toner particles. Electrostatic transfer of conductive toner particles to plain bond paper is, however, usually inefficient as the charge on the toner particles can be reversed by induction charging from the paper during the transfer step. Accordingly, electrophotographic systems wherein conductive single component toner particles are used require an alternative transfer method and materials such as a special overcoated insulating paper to achieve sufficient toner transfer. Furthermore, in single component systems with conductive toner particles, the control of undesirable background or background suppression cannot usually be achieved with electrostatic forces alone, as the toner particles are inductively charged and deposited on the image bearing member in both the image and background areas, which is not the situation in two component developer systems where suppression of background development is accomplished by electrostatic forces acting on the triboelectrically charged toner particles, causing such particles to be directed away from image bearing member.
Recent developments in the area of single component development have resulted in an efficient, economical, simple process and apparatus wherein insulative, non-magnetic, toner particles are appropriately charged and there is obtained two component image quality utilizing a single component development apparatus. In this system, as described in U.S. Pat. No. 4,505,573 issued on Mar. 19, 1985, the disclosure of which is incorporated herein by reference, a charging roll means simultaneously meters and charges toner particles. A donor electrode roll serves to transport the toner metered and charged particles from the charging roll to a charge retentive surface. The electrode can be comprised of numerous suitable materials including for example a conducting roll overcoated with a polymer containing carbon black.
Reloading of nonmagnetic single component development systems of prior devices is ineffective, particularly, in the case of developing continuous solid areas. Reload refers to the capability to restore the donor roll with toner in a single pass even under the stress condition of developing continuous solid areas. Adequate toner supply, flow and charging are requirements for reload.
In one prior art device, toner is transported down the length of a developer housing with a rotating toner mover which fluidizes toner in the developer sump. A DC bias (-1000 volts) between the toner mover and donor assists in loading right sign toner on the donor in the prenip region. Since the gap between the toner mover and donor is relatively large (0.06 inch), the applied electric field is low. Significant effort has been devoted to optimizing the toner mover design. Recent toner movers (holey tube, star, paddlewheel) show equivalent performance, but fall short of adequate reload. Typically with these toner movers, one observes reload defects such as loss of density or nonuniform density within the first three copies of a continuous solid area.
Certain patents which may be relevant to the present invention will now be discussed.
U.S. Pat. No. 4,382,420 issued on May 10, 1983 relates to an apparatus for developing a latent electrostatic image formed on a photoconductive recording material in a dry type electrophotographic copying machine typically employing a one component type developer with a conductive electrode held in contact with the developer. The electrode is connected to a power source through a switching device and serves to charge the developer to a predetermined polarity with a predetermined potential before the latent image is developed. In this way, the latent image can be developed selectively as either a normal image or a reverse image quite easily. FIG. 6 thereof discloses a charge and metering roller which regulates the layer of toner on a development belt and serves as an electrode as well.
U.S. Pat. No. 4,459,009 issued on Jul. 10, 1984 relates to a process and apparatus for charging insulating toner particles wherein there is provided a charging roll containing a triboelectrically active coating, and weakly charged toner particles are transported into contact with the coating contained on the charging roll, this contact being accomplished in a charging zone situated between the charging roll and the transporting mechanism. As a result of contact between the weakly charged toner particles and the triboelectrically active coating contained on the charging roll there is imparted charges of either a positive or negative polarity to the weakly charged toner particles. The apparatus and process of the present invention are useful, for example, in electrostatographic recording imaging devices.
U.S. Pat. No. 4,868,600 granted on Sep. 19, 1989 to Hays et al discloses a scavengeless development system in which toner detachment from a donor and the concomitant generation of a controlled powder cloud is obtained by AC electric fields supplied by self-spaced electrode structures positioned within the development nip. The electrode structure is placed in close proximity to the toned donor within the gap between the toned donor and image receiver, self-spacing being effected via the toner on the donor. Such spacing enables the creation of relatively large electrostatic fields without risk of air breakdown.
U.S. Pat. No. 4,876,575 granted on Oct. 24, 1989 to Dan A. Hays discloses a development apparatus including structure for the dynamic toner metering and charging of nonmagnetic single component toner. To this end there is provided a flexible, rotating rod having an electrical bias applied thereto. The rod is captured or supported by a distributed bearing attached to a compliant blade. A toner cleaning blade held against the rod serves as a toner seal. The flexible rod is supported in a self-spaced relationship to a rigid donor roll which transports the charged toner to a development zone intermediate the donor roll and an imaging member. Self-spacing is provided by a layer of toner on the donor structure. The donor roll and flexible rod form a toner metering and charging zone through which toner is moved to simultaneously charge and meter the toner particles. The roll and flexible rod are rotated in opposite directions for controlling the metering and charging of the toner in the nip
U.S. patent application Ser. No. 07/428,726 filed Oct. 30, 1989 in the name of Brewington et al and assigned to the same assignee as the instant application discloses an apparatus which develops a latent image recorded on an image receiving member with developer material. A chamber in the developer housing stores a supply of developer material. A donor roll is positioned in the chamber of the housing so as to transport developer material into contact with the latent image to develop the latent image. A rotating, elongated member fluidizes the developer material. As developer material is discharged from a storage container into the chamber of the developer housing, it exerts pressure on the fluidized developer material to move the developer material from one end of the housing to the other end thereof. An electrical bias is applied between the elongated member and the donor roll so that developer material is attracted to the donor roll as the developer material advances from one end of the developer housing to the other end thereof.